The present application relates generally to apparatus and methods of measuring the shape of a semiconductor wafer, and more specifically to apparatus and methods of measuring semiconductor wafer shape with increased accuracy.
In the manufacture of semiconductor devices, the shape of a semiconductor wafer is typically measured to determine whether the wafer conforms to predetermined criteria or standards. Such standards may relate to various wafer shape parameters including flatness, bow, and warp. In the event it is determined that a semiconductor wafer fails to conform to certain wafer shape standards, the wafer may be deemed unusable and subsequently discarded.
For example, during a semiconductor manufacturing process, semiconductor wafers may be held by wafer handling equipment in a horizontal position or in a vertical position during wafer testing and/or measurement. Such wafers are often held in the vertical position to reduce sagging of the wafer due to gravity, and to reduce wafer contamination by minimizing any horizontal surfaces of the wafer upon which particles in the air might settle. However, if a semiconductor wafer has excessive bow or warp, then results obtained from wafer testing or measurement may be inaccurate. In addition, such excessive bowing or warping of the semiconductor wafer may cause unwanted stresses to develop on thin film layers that may have been previously deposited on the wafer surface.
Not only may a semiconductor wafer exhibit excessive bow or warp, but the wafer handling equipment itself may exacerbate the bowing or warping of the wafer. For example, conventional wafer handling equipment typically includes wafer gripping fingers having V-shaped grooves configured to hold a wafer by its edges. The grooves of the wafer gripping fingers are typically made of polyetheretherketone (PEEK), which is a non-contaminating material used in various applications in the semiconductor industry. Because the V-shaped grooves may over-constrain the wafer and cause indeterminate loading at the groove/wafer contact surfaces, the grooves of the wafer gripping fingers may distort the wafer shape. For example, when the V-shaped grooves are used to hold a semiconductor wafer in a vertical position, a torque may develop that twists the wafer, thereby increasing the error in wafer shape measurements. Moreover, such loading-induced measurement error is often a function of the original shape of the wafer being measured. For this reason, it can be difficult if not impossible to separate the loading-induced error from the wafer shape parameter being measured.
It would therefore be desirable to have an apparatus and method of holding a semiconductor wafer in a wafer test or measurement station that precisely positions the wafer while minimizing any distortion in the wafer shape. Such an apparatus would position the wafer in the wafer test/measurement station so that the wafer lies within the range of the measuring instrument over a range of wafer shape parameters. It would also be desirable to have an apparatus and method of holding a wafer in a wafer test/measurement station that not only minimizes distortion of the wafer, but also provides robust wafer gripping force.